Recent years, digital radiation image detectors represented by, for example, computed radiography (CR) and flat panel detectors (FPD) have been broadly used in image diagnosis in hospitals, clinics and the like because they allow radiation images to be obtained directly and are capable of displaying the images directly on an image-displaying apparatus such as a cathode tube or a liquid crystal panel. Recently, flat panel which utilizes a scintillator layer containing cesium iodide (CsI) and is combined with a thin film transistor (TFT) have attracted attention as a highly sensitive X-ray image visualization system.
In such X-ray imaging, when X-rays are irradiated onto a subject, the incident radiation gets slightly scattered, depending on the subject as well as a plate member and a case present as necessary on a radiation incident side.
As a result, on a scintillator portion where the dose of incident radiation would be large otherwise, the dose of the incident radiation becomes smaller because of the scattering. On the other hand, scattered radiation enters a scintillator portion where the dose of incident radiation should be small without the scattering, causing a phenomenon in which the dose of incident radiation on the portion becomes larger.
Consequently, radiation images taken in such a situation have caused a problem such as the decrease in contrast due to the weak scattered radiation and the reduced quality of radiation images due to the occurrence of fogging and blurring. Such radiation images have been liable to interfere with precise diagnosis by use of radiation images because, for example, they do not help measure the exact size of a lesion site captured in the radiation images and make it difficult to determine if the lesion site is becoming bigger or smaller.
Conventionally, as a method for removing such scattered X-rays, it has been known to locate a plate in which a lead foil is arranged in slits or in a lattice shape (referred to as grid, and a method for removing scattered X-rays using such a grid is called grid method) between a subject and a detector (scintillator) and thus absorb and remove the scattered X-rays. However, there has been a problem in which a larger dose of radiation is required since even X-rays necessary for the imaging are also absorbed.
As another method for removing scattered X-rays, air gap method is also known. In the air gap method, the subject is located closer to the radiation source, and a distance of several tens of centimeters is placed between the scintillator and the plate member, forming an air gap therebetween. Then, the scattered X-rays from the subject are diffused and are more unlikely to reach the scintillator. In this method, although the contrast improves, there has been a problem of a smaller area available for the imaging due to the approach of the subject to the radiation source and of the decrease in sharpness because of larger blurring along with the expansion of the image.
The applicant of the present invention proposes, in JP 2014-142217 A (Patent Document 1), a radiation image forming apparatus capable of removing scattered X-rays by use of a radiation absorbing layer and preventing the decrease in contrast of radiation images, wherein the radiation absorbing layer is provided within the range of 0 to 0.5 mm from a radiation incident side of a phosphor of the scintillator towards the plate member.
However, when a layer that absorbs a large amount of radiation is present, more radiation than in a conventional case is required in order to obtain the same image quality as when radiation is not absorbed. Originally, it is desired that the examination is performed only on a necessary site at a minimum dose of X-rays for less influence of radiation.
Therefore, a scintillator panel has been demanded which is capable of imaging at a low dose while suppressing the contrast deterioration caused by scattered radiation, and further has improved radiation image characteristics.